Here's a quick way to feed a crowd. The salmon is also great made ahead and served chilled.
2/3 cup mayonnaise
½ cup spicy brown mustard (such as gulden's)
6 tablespoons chopped fresh tarragon
6 tablespoons chopped fresh chives
2 tablespoons (packed) golden brown sugar
2 tablespoons fresh lemon juice
2 teaspoons finely grated lemon peel
1 3½-to-3¾-pound whole side of salmon with skin (about 1½ inches thick at thickest part)
Position rack in center of oven and preheat to 450°F. Line large rimmed baking sheet with parchment paper. Mix first 7 ingredients in medium bowl. Season mustard sauce lightly with salt and and generously with pepper. Place salmon, skin side down on diagonal on prepared sheet. Spoon ½ cup mustard sauce atop salmon, then spread over, covering completely. Sprinkle salmon generously with salt and pepper. Roast just until salmon is opaque in center, about 15 minutes. Using parchment as aid, transfer salmon to platter. Cut crosswise into pieces and serve with remaining mustard sauce.